1. Field of the Invention
The present invention relates, in general, to methods for etching a substrate in self alignment and, more particularly, to use of an electropolishing system employing a constant current and a chemical etchant.
2. Description of the Prior Art
Sometimes, development in micromachine technology has partly been based on a silicon etch process for fine structure. For example, in an ink jet printer, such as that manufactured by Hewlett Packard Co. Ltd., a printer head is provided with a substrate having holes which aim to supply ink from an ink reservoir to an ink nozzle. Another example is a microaccelerometer for auto airbag, which measures an acceleration upon impact by sensing a distortion of cantilever. In the microaccelerometer, a base portion for cantilever and its circumferential substrate must be eliminated, in order to secure a sufficient vibration amplitude of the cantilever. Such holes in the substrate are ordinarily formed in the last step of the process, therefor. The reason is as follows.
Photolithography is necessarily required to determine a fine pattern. If the holes of the substrate were formed in the initial or middle steps of the process, a photosensitive film might nonuniformly be coated due to the surface tension generated at the brink of the hole. Accordingly, it might be virtually impossible to form a pattern for a subsequent structure. This order limitation in the hole formation results in setting limits to the formation method of hole, thus deteriorating the quality of the resulting hole. In addition, the production cost of the fine structure rises, but the performance thereof is lowered.
Following are of the detailed reasons for the conventional problems. Generally, a silicon substrate is etched using calcium hydroxide or a mixture solution consisting of ethylene diamine, pyrocatechol and water or of hydrofluoric acid, nitric acid and acetic acid. These chemicals are able to etch silicon substrates only at a rate of several microns per minute. For the formation of holes, silicon substrates thus should be immersed in the chemicals for several hours. However, the chemicals may destroy other ready-formed structures because of their strong corrosive action. Therefore, it is required that the silicon substrate is additionally pre-treated to protect the ready-formed structures, in advance of forming the holes in the silicon substrate by use of the chemicals.
Another principal problem of the prior arts is that it is very difficult to accurately determine the final size of the holes in the upper surface of the substrate as the silicon substrate is considerably thick, for example, about 500 microns thick. In addition, due to a fact that the formation of holes starts at the bottom of the silicon substrate, there is undertaken complicated yet coarse processes that an infrared alignmeter is used or an alignment pattern should be previously placed on the bottom of the substrate, in order to align the holes with other ready-formed structures.
To solve the above-mentioned problems, Hewlett Packard Co., Ltd. has made an attempt to use laser drilling and sand impact on the upper surface of the substrate to form holes in the substrate. However, there are still disadvantages, such as inevitable dust generation and thermal and mechanical impact on the substrate. In addition, the Hewlett Packard's method is unable to form fine holes in the silicon substrate. Furthermore, no structure can be formed in an area which is destined to have holes. And, the laser equipment is expensive, which has an adverse influence on the production cost.